Important: #106494 - Adapt custom instances of AbstractFormFieldViewHelper to deal with persistenceManager->getIdentifierByObject() methods

See forge#106494

Description

When dealing with relations to multilingual Extbase entities, these relations should always store their reference to the "original" (sys_language_uid=0) entity, so that later on, language record overlays can be properly applied.

For this to work in areas like persistence, internally an "identifier" is established that references these multilingual objects like [defaultLanguageRecordUid]_[localizedRecordUid].

Internally, this identifier should be converted back to only contain/reference the defaultLanguageRecordUid.

A bug has been fixed with #106494 to deal with this inside the <f:form.select> ViewHelper, which utilized an <option value="11_42"> (defaultLanguageRecordUid=11, localizedRecordUid=42), and when using an <f:form> to edit existing records, the currently attached records would NOT get pre-selected.

When such objects with relations were persisted (in frontend management interfaces with Extbase), if the proper option had not been selected again, the relation would get lost.

Important: Adapt custom ViewHelpers extended from AbstractFormFieldViewHelper or using persistenceManager->getIdentifierByObject()

The bug has been fixed, but it is important that if third-party code created custom ViewHelpers based on \TYPO3\CMS\Fluid\ViewHelpers\Form\AbstractFormFieldViewHelper , these may need adoption too.

Instead of using code like this:

Example ViewHelper code utilizing persistenceManager->getIdentifierByObject() 
if ($this->persistenceManager->getIdentifierByObject($valueElement) !== null) {
    return $this->persistenceManager->getIdentifierByObject($valueElement);
}
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the code should be adopted to not rely on getIdentifierByObject() but instead:

Refactored ViewHelper code preferring an object's getUid() method instead 
if ($this->persistenceManager->getIdentifierByObject($valueElement) !== null) {
    if ($valueElement instanceof DomainObjectInterface) {
        return $valueElement->getUid() ?? $this->persistenceManager->getIdentifierByObject($valueElement);
    }
    return $this->persistenceManager->getIdentifierByObject($valueElement);
}
Copied!

This code ensures that retrieving the relational object's UID is done with the overlaid record, and only falls back to the full identifier, if it's not set, or not an object implementing the Extbase DomainObjectInterface.

Also note that the abstract's method convertToPlainValue() has been fixed to no longer return a value of format [defaultLanguageRecordUid]_[localizedRecordUid] but instead always use the original record's ->getUid() return value (=defaultLanguageRecordUid).

If this method convertToPlainValue() is used in 3rd-party code, make sure this is the expected result, too.